Polyethylene composition and
process of crosslinking



United States Patent 25,941 POLYETHYLENE COMPOSITION AND PROCESS OFCROSSLINKING Razmic S. Gregorian, Silver Spring, Md., assignor to W. R.Grace & Co., New York, N.Y., a corporation of Connecticut No Drawing.Original No. 3,118,866, dated Jan. 21,

1964, Ser. No. 946, Jan. 7, 1960. Application for reissue Jan. 21, 1965,Ser. No. 441,942

9 Claims. (Cl. 260-949) Matter enclosed in heavy brackets appears in theoriginal patent but forms no part of this reissue specification; matterprinted in italics indicates the additions made by reissue.

This invention relates to a novel method of crosslinking olefinpolymers. More particularly, this invention is concerned withcrosslinking polyethylene by subjecting ethylene polymer to the actionof a novel class of peroxides at a temperature above the softening pointof the polymer.

This invention is a continuation-in-part of my copending applicationhaving Serial Number 757,918, now abandoned.

Polymers of ethylene such as those described in US. 2,153,553 and in US.2,816,883 are well known in the art today and are generallycharacterized by their organic solvent solubility and theirthermoplastic properties. Lately, several methods have been tried withvarying success to decrease the thermoplasticity and solubility bycrosslinking the polymer. Such methods include electron impingement ofthe polymer and blending of free radical liberating substances into thepolymer. In regard to the aforesaid latter method of polymercrosslinking the main class of compounds which have been utilized isorganic peroxides. Although it is known that various organic peroxideswill cause crosslinking upon admixture with polyethylene, many have beenfound lacking in certain regards. Thus, for example, benzoyl peroxide atthe necessary blending temperatures is a hazard due to its capability todecompose violently. Still other peroxides are characterized by failingto have a decomposition temperature which is high enough to allowdecomposition to take place at or above the softening point of theethylene polymers. This is especially true of the high densitypolyethylene described in US. 2,816,883 which has a melting point of 127C. or higher. Still other peroxides lack a suflicient half-life at thetemperatures of incorporation into the softened ethylene polymer topermit uniform crosslinking therein.

it has now been found that bisperoxidcs having the general formula:

wherein R is a member of the group consisting of methyl and phenyl, areeffective crosslinking agents for promoting the crosslinking ofpolyethylene. These bisperoxides promote crosslinking to a high degreeat temperatures above the softening point of the ethylene polymeremployed. The crosslinked polymer exhibits improved clarity andenvironmental stress cracking properties as will be shown hereinafter inthe examples.

According to the instant invention the bisperoxides are operable withboth high and low density polyethylene, i.e., having a density in therange 0.92-0.96.

summarily, this invention relates a method of crossreactants anddetermine the degree of crosslinking.

Re. 25,941 Reissued Dec. 14, 1965 "ice linking polyethylene whichcomprises contacting a minor amount of a bisperoxide of the formula:

wherein R is a member of the group consisting of methyl and phenyl witha polymer of ethylene and crosslinking said polymer at a temperatureabove its softening point.

The following examples are set down to illustrate the invention and arenot to be deemed as limiting its scope. Throughout the instant inventionthe melt indices (M1) were measured under the conditions specified inASTMD 1238-521, the densities of the polymer were measured in a densitygradient tube by the Bell Laboratories Proposed ASTM Method for theMeasurement of Density of Solid Plastics by the Density GradientTechnique, and the environmental stress-cracking data (ESC) was obtainedusing Igepal CO63O (Antarox A400) an alkylaryl polyethylene glycolproduced by General Dyestuff Corp., in accordance with the ProposedTentative Method of Test for Environmental Stress-Cracking of Type IEthylene Plastics (ASTM Designation: D00-59T) as disclosed in the 1959preprint of the Report of Committee D20 on Plastics, pp. 17-22 at the62nd Annual Meeting of the ASTM, June 21-26, 1959. The clarity testdevised specifically for evaluation of changes in the clarity ofpolyethylene due to crosslinking consists of viewing. through a V2"diameter hole centered in a horizontal sample table holding clarity testsamples thereon, a glowing filament from a 2-watt concentrated arc lamp(point light source) the extreme tip of said filament being situated 3inches below the bottom surface of said test sample. The test samplesare prepared by molding under 10,000 lbs. pressure at 350 F. and thenair cooled to room temperature. The results of the clarity test areexpressed in terms of the clarity number (mils) which is defined as themaximum thickness of the sample in mils through which the glowingfilament, as viewed from 1 foot above the sample table, can still beobserved.

In all examples, unless otherwise noted, a Brabender Plastograph ModelPLV2 equipped with a recording unit for measuring changes in torque wasused to admix the The aforesaid recording unit has a range of 0-1000units equal to 01 kilogram-meter of torque. This range can be increasedwhen necessary to 0-5000, i.e., equal to 05 kilogram-meters of torque bythe addition of weights. However, other mechanisms, e.g., a Banburymixer or a tape extruder, are equally operable in performing thisinvention.

The degree of crosslinking is related to the increase in torque (Ar)measured by the Plastograph recorder from the time the bisperoxidecrosslinking agent is added to the fused polymer until the crosslinkingreaction is discontinued. The greater the degree of crosslinking thegreater the viscosity of the polyethylene which in turn requires agreater torque in order to drive the Plastograph at a constant rpm. Thedegree of crosslinking which can be accomplished by the instantinvention is limited only by the ability of the mixing apparatus toovercome the torque caused by the crosslinking. Thus, by the presentinvention, polyethylene can be wholly crosslinked, if desired, underproper conditions and with proper equipment available.

A further check on the degree of crosslinking is the decrease in meltindex due to crosslinking of the polyethylene since melt index variesinversely with viscosity which varies directly with degree ofcrosslinking, a lower melt index after crosslinking evidences thatcrosslinking occurred.

Unless otherwise noted, all parts and percentages are by weight in theexamples.

EXAMPLE 1 in benzene was added to the fused polymer and millingcontinued for 2 minutes. A1- Was 3780 units. The crosslinkedpolyethylene on characterization had a melt index of 0.07.

EXAMPLE 2 38 g. commercial polyethylene in granule form having a meltindex of 0.7 and a density of 0.96 were charged over a 4 minute periodto a Brabender Plastograph maintained at a temperature of 150 C. After14 minutes 1.0 cc. of a 52% solution of a,m,a',a'-tetramcthylisophthalyl di-cumyl bisperoxide:

in benzene was added to the fused polyethylene and milling was continuedfor an additional 7 minutes at which time Ar was 520 units. Thethus-crosslinked polyethylene had a melt index of 0.00.

The crosslinking step of the instant invention is carried out attemperatures above the softening point. Temperatures up to 200 C. oreven higher are operable so long as heat degradation of the polymerwhich is a function of temperature and time at temperature does notbecome a factor. For low density polyethylene (i.e., 0.92) a preferabletemperature range for the crosslinking step is IOU-200 C. and for highdensity polyethylene (i.e., 0.96) a preferable range is 150200 C.

The amount of bisperoxide addition necessary to crosslink the ethylenepolymer is relatively minor. Amounts in the range of 0.001-10.0% basedon the weight of the polyethylene charge are operable. A preferablerange is 0.001 to 0.5% based on the weight of the polymer charge.

It should be understood that the bisperoxide can be added to thepolyethylene and blended therewith at temperatures below the softeningpoint of the polymer. However, it is preferable to add to the fusedpolymer to obtain more uniform dispersion prior to decomposition of thebisperoxide.

The bisperoxides operable in this invention can be prepared by methodswell known in the art. For example the acid method described in JACS 68,p. 1938 (1946), is preferably used to prepare ditertiary bisperoxides ofthe general formula:

CH3 pm (7H3 pm l l i i wherein R is a member of the group consisting ofmethyl and phenyl.

The crosslinked polyethylene produced by the instant invention can beused in many applications wherein polyethylene was used heretofore. Thecomposition of the present invention is especially useful wherepolyethylene with improved clarity is necessary.

I claim:

1. A composition useful in the production of crosslinlted polyethyleneconsisting essentially of polyethylene and 0.00ll0% by weight of saidpolyethylene of a crosslinking agent of the formula wherein R is amember of the group consisting of methyl and phenyl.

2. The composition according to claim 1 wherein the crosslinking agentis a,a, x',a-tetramethyl isophthalyl di-t-butyl bisperoxide.

3. The composition according to claim 1 wherein the crosslinking agentis a,u,a,a'-tetramethyl isophthalyl di-cumyl bisperoxide.

4. The process which comprises admixing polyethylene and 0.001-10% byweight of said polyethylene of a crosslinking agent of the formula C11(3 ll;

wherein R is selected from the group consisting of methyl and phenyl,and heating said admixture to effect crosslinking of said polyethylene.

5. The process according to claim 4 wherein the crosslinking agent ism,a,a',a'-tetramethyl isophthalyl di-t-butyl bisperoxide.

6. The process according to claim 4 wherein the crosslinking agent isa,nr,a',u-tetramethyl isophthalyl di-cumyl bisperoxide.

7. The process according to claim 4 wherein the crosslinking is effectedat a temperature above the softening point of the polyethylene.

8. A composition useful in the production of crosslinked polyethyleneconsisting essentially of polyethylene and 0.001-10% by weight of saidpolyethylene of a crosslinking agent of the formula wherein R isselected from the group consisting of methyl and phenyl, and heatingsaid admixture Io eficct crosslinking of said polyethylene.

References Cited by the Examiner The following references, cited by theExaminer, are of record in the patented file of this patent or theoriginal patent.

UNITED STATES PATENTS 2,8l3,l27 ll/l957 White 260-6l0 2,826,570 3/1958Ivett 26094.9 2,888,424 5/1959 Prccopio 26094.9 2,916,481 l2/l959Gilmont 26094.9 2,957,030 10/1960 Bankert 260--6l0 JOSEPH L. SCHOFER,Primary Examiner.

MORRIS LIEBMAN, L. H. GASTON, Examiners.

